Up until now I have focused on non-chemical control methods for cotton aphid, induced defense may be a safe and useful method, but I don't know what kind of substance could induce cucumber defense cotton aphid?
Methyl Salicylate (MeSA) has been used in many crop plants to induced plant defenses against phloem-feeding insects like aphids. Serveral publications had documented evidences of this induction of defenses indeed translated into induced resistance against insect pest. I think you can use this on Cotton and cucumber as well, but I haven't read any paper about the effect of MeSA on cotton or cucumber. The commercial version of MeSA for agricultural use is called Actigard. Good luck